1. Field of the Invention
This invention relates to the field of pigment free coating compositions and particularly to such compositions which are suitable for providing abrasion resistant coatings to plastic substrates. The composition of the invention is particularly suited for the coating of optic lenses wherein the cured coating composition exhibits excellent adhesion to the plastic surface.
2. Description of the Prior Art
In recent years there has been considerable interest in the formation of materials out of plastic which were formerly made from glass, in order to provide the shatter resistant characteristics which are provided by plastics. Examples of such use include, for example, large plastic sheets for use as window panes, decorative mirrors for use in buildings and other architectural panels, various parts of automobiles, such as reflectors and windshields, lighting signs, packaging in aircraft, as machine guards, view ports, particularly in the field of safety glasses, helmets, face guards and opthalmic or optical lenses.
The plastic materials which are available for the above uses must be pigment-free with a high visible light transmission after molding or casting. Examples of such plastics include, among others, acrylics, polyesters, polycarbonates, polyamides, copolymers of acrylonitrile-styrene, styrene-acrylonitrile-butadiene copolymers, polyvinylchlorides, butyrates, polyethylene, and the like.
The above mentioned plastics are all characterized by varying degrees of impact resistance, weatherability, chemical resistance, hardness and other desirable qualities which would be selected on the basis of their final use. It has been found that all of these plastics suffer from the undesirable characteristic of varying degrees of scratch and abrasion resistance.
In an attempt to overcome these undesirable characteristics of the plastics, various coating compositions have been tried in an effort to impart improved abrasion resistance to the surface of these materials. Perhaps the best available composition which has been used is a pigment-free coating composition, which is the subject of U.S. Pat. No. 3,986,997 to Harold A. Clark. This composition combines a colloidal silica hydrosol containing very small particles of colloidal silica in the range of about 10 to 30 millimicron particle size with a lower aliphatic alcohol-water solution of the partial condensate of a silanol, preferably monomethyltrisilanol, and sufficient acid to give a pH in the range of about 3 to about 6. The composition contains approximately 10 to about 50 weight percent solids of which 10 to 70 percent is comprised of the colloidal silica and 30 to 90 weight percent is comprised of the silanol partial condensate. The solvent system for the above mentioned composition includes from about 20 to about 75 weight percent alcohol with preferably 50 weight percent isopropanol in the mixture. Water is present in the composition through the silica hydrosol and through condensation of the silanol.
The composition as above described is coated onto plastic materials by any conventional method, such as by flowing, spraying, or dip-coating to form a continuous surface film. After coating, the plastics are air dried and then cured by baking at a temperature which is consonant with the plastic material which is to be coated.
The coating thus formed on the plastic is characterized by excellent abrasion resistance, but suffers from a very serious drawback in that adhesion of the coating to the plastic material is very difficult to obtain. Adhesive problems are related to the type of plastic and are particularly acute when the plastic material to be coated is polycarbonate. Polycarbonate plastic is particularly favored for the production of optic lenses, due to its clarity coupled with particularly high impact resistance, chemical resistance, and toughness. Unfortunately, it is quite difficult to obtain adherence between polycarbonate plastics and the above described abrasion resistant coating composition.
The problem of adhesion to polycarbonate plastics is well recognized as discussed in the U.S. Pat. No. 3,986,997. It has been suggested that such adhesion might be improved by pretreatment of the polycarbonate plastic surfaces by overnight soaking in an alkaline solution, for instance a ten percent aqueous solution of potassium hydroxide. This step has not been found to be successful in providing the desired degree of adhesion of the abrasion resistant coating on the polycarbonate surface.
It has also been suggested to improve adhesion by priming polycarbonate with a five percent solution of a silane-modified epoxy in "Dowanol-EM" and allowing to air dry. The recommended epoxy primer is a mixture of about twenty percent beta-aminoethylgamma-aminopropyltrimethoxy silane, in a commercially available liquid epoxy sold under the trademark "DER-331" by the Dow Chemical Company. This priming step, which acts as a coupling agent between the plastic substrate and the abrasion resistant coating has not been found to provide satisfactory adhesion between the coating composition and the polycarbonate.
Satisfactory adhesion is measured by applying a strip of "3 M Scotch" No. 650 brand adhesive tape onto a one eigth inch cross hatched grid of the cured coating and rapidly pulling the tape up and off of the plastic surface. Where there is a problem in adhesion, it can be readily seen where the coating has been pulled off of the surface of the lens.
The fact that the plastics to be coated are valued for their clarity, or light transmission, makes the surface preparation of such plastics prior to application of an abrasion resistant coating a difficult one. Any surface preparation which is to be done to the plastics prior to coating with the abrasion resistant composition must be of a minimal nature so that the light transmission for which the plastic is selected, is not interfered with. This can be particularly serious when such plastics are exposed to chemicals which etch the plastic or attack the surface of the plastic. Additionally, any priming coating, such as the epoxy silane mixture mentioned above, must be of a clear pigment-free nature which will not cause a reduction in the clarity of the final coated lens. In effect, the commonly used methods for improving adhesion between dissimilar surfaces, such as roughening the surface or the application of coupling agents is fairly limited.
Until the present time, there has been no satisfactory means of improving the adhesion properties of the coloidal silica-silanol partial condensate described above, without interfering with the light transmission of the plastic material to be coated.